The term “hematocrit” (Hct) refers to the proportion of a blood volume occupied by red blood cells (RBCs). The RBCs contain hemoglobin (approximately 34% by volume), which enables the delivery of oxygen from the lungs to the various tissues in the body. The normal ranges of hematocrit for an individual varies with age and gender, but is generally around 42-54% for adult men and 38-46% for adult women. The hematocrit measurement of a patient is useful for various medical diagnoses. A low hematocrit usually indicates anemia (a deficiency in RBCs and hemoglobin). Anemia may result from acute blood loss (e.g., traumatic injury, surgery, colon cancer), nutritional deficiency (e.g., iron, folic acid, vitamin B-12), or various other clinical factors (e.g., cancer, medication effects, hereditary disorders, etc). An elevated hematocrit may be due to dehydration, or may indicate other conditions, such as a myeloproliferative disorder (type of bone marrow disorder) or chronic obstructive pulmonary disease and other conditions associated with hypoxia (oxygen deficiency in the body).
The hematocrit is usually measured with invasive techniques. In one common technique, blood is drawn from the patient, and then a high speed centrifuge is used to separate the different blood components into separate layers inside a capillary tube. The relative height of the RBC layer in the tube is measured relative to the total blood volume, to provide the hematocrit value. The drawn blood may also be analyzed by other methods, such as by taking optical measurements of the blood. The hematocrit value may be calculated using an automated instrument, as part of a complete set of blood tests.
The aforementioned invasive measurement involves inconvenience and discomfort for the subject, and also introduces the risk of contamination to both the subject and the medical staff performing the measurement. Additionally, any invasive method (such as the centrifuging technique) entails a delay between when the blood sample is drawn and when the hematocrit value is obtained. Moreover, the determination of hematocrit (as well as other types of blood tests) is usually based on individual measurements. In certain situations, it is beneficial to perform continuous monitoring of the hematocrit of a subject, to effectively diagnose and treat certain serious or life-threatening medical conditions (e.g., during intensive care, or when performing surgery). However, if an invasive measurement technique is being used, it is not possible to obtain immediate results, nor can continuous real-time tracking of the hematocrit of the subject be performed.
Pulse oximetry is a non-invasive method for measuring the level of oxygen saturation in the hemoglobin of a patient. A hemoglobin molecule bounded with oxygen is known as oxyhemoglobin (HbO2) and a hemoglobin molecule without bound oxygen is known as deoxyhemoglobin (Hb). Pulse oximetry involves emitting light at two or more separate wavelengths (e.g., red and infrared light) across a body part, such as a fingertip. The total attenuation of each of the two wavelengths is measured using sensors (i.e., based on reflectance or transmittance measurements). Pulse oximetry utilizes the pulsing nature of the blood vessels, in order to isolate the blood in the artery from other tissues which the light passes through (e.g., skin, bone, muscle, fat, fingernail, etc). The various blood vessels in the body change volume in a cyclic manner, due to the pumping action of the heart as the blood circulates throughout the body. The ratio between oxyhemoglobin and deoxyhemoglobin components in the blood is determined based on the different absorption spectra of oxyhemoglobin and deoxyhemoglobin. It is noted that pulse oximetry only reveals the percentage of oxygen saturation in the hemoglobin (i.e., the percentage of oxyhemoglobin relative to deoxyhemoglobin), and provides no information regarding other blood parameters, such as the total oxygen content in the blood, the amount of oxygen dissolved in the blood, or the absolute hematocrit value. Therefore, pulse oximetry cannot be used to diagnose various disorders, such as anemia or states of elevated hematocrit (for instance, the existing RBCs may be fully oxygenated, but there may not be sufficient blood cells).
Contemporary approaches for the noninvasive measurement of blood parameters such as hematocrit are usually based on the concept implemented in pulse oximetry of isolating the blood from the other tissues, by analyzing the total attenuation of light passing through the body part of the subject due to the blood in the body tissue. However, even after isolating the blood component, the total light attenuation is a function of at least two unknown parameters: the total blood volume and the hematocrit value, which are each independent absolute values (unlike the ratio measurement which provides the hemoglobin oxygenation). Therefore, an additional measurement is required in order to be able to determine both parameters. For example, an additional measurement relating to the total amount of blood can be carried out to extract its relationship to the total attenuation, and thereby solve for the hematocrit value.
Other methods that aim to isolate the blood from other tissues are based on physiological properties of RBCs in certain conditions, such as occlusion, in which cessation of blood flow is established at the measurement location (e.g., by applying over-systolic pressure at a location upstream of the measurement location with respect to the direction of normal blood flow). Such methods are described in U.S. Pat. No. 6,213,952 to Finarov et al, entitled “Optical device for non-invasive measurement of blood related signals using a finger holder”, and U.S. Pat. No. 6,587,704 to Fine et al, entitled “Method for non-invasive optical measurements of blood parameters”.
U.S. Pat. No. 6,662,031 to Khalil et al, entitled “Method and device for the noninvasive determination of hemoglobin and hematocrit”, discloses the determination of the hemoglobin concentration and hematocrit value of a human tissue sample based on steady state reflectance measurements, and the localized control of the temperature of the tissue sample. The body part is set to a particular temperature within a physiological temperature range (below the core temperature of the body), and the skin surface is illuminated by light at wavelengths within the spectral range of about 400 nm to about 900 nm. Detectors positioned at particular separation distances collect the reflect light (e.g., using spatially resolved diffuse reflectance measurement techniques), while the temperature is being maintained. A second set of measurements are acquired while the body part is set to another temperature. Optical parameters are determined at each temperature, along with the temperature dependence of the optical parameters. The hemoglobin concentration or the hematocrit value is determined based on a calibration relationship that relates the optical parameters at a given temperature, and the dependence of the optical parameters on temperature with the hemoglobin concentration or hematocrit value.
U.S. Pat. No. 6,671,528, and the continuation U.S. Pat. No. 6,873,865 to Steuer et al, both entitled “Method and apparatus for non-invasive blood constituent monitoring”, describe the determination of blood hematocrit in a living tissue based on an attenuation measurement and an energy measurement. Radiation at a selected wavelength is directed toward a body part, and detectors obtain transmittance or reflectance measurements. An energy-detecting means (e.g., a pressure transducer) measures the temporal rate of change of the fractional blood volume (using one of several possible techniques). The hematocrit value is calculated from the attenuation measurements (from the detectors) and the energy measurements (from the energy-detecting means).
U.S. Pat. No. 6,681,128 to Steuer et al, entitled “System for noninvasive hematocrit monitoring”, is directed to a method and system for noninvasive determination of the hematocrit and other blood parameters of a subject. The method involves passing at least two wavelengths through a body tissue and detecting the transillumination. The selected wavelengths are preferably at isosbestic points of reduced hemoglobin and oxyhemoglobin, to eliminate the effects of variable blood oxygenation. The hematocrit is calculated based on a ratio of extinction coefficients, in terms of the pulsatile component and the steady state component for each wavelength that is extinguished after passing through the body tissue.